Deep brain electrical stimulation is a therapeutic technique comprising the implantation of a medical device known as a brain stimulator that serves to send electrical pulses to specific portions of the brain, by means of leads for electrical stimulation implanted in the appropriate neural structures. For example, stimulating the thalamic nuclei or the subthalamic nuclei can be useful in treating motor disorders such as tremor as caused in particular by Parkinson's disease (see the article by A-L. Benabid, P. Pollak, C. Gervason, D. Hoffmann, D-M. Gao, M. Hommel, J-E. Perret, and J. de Rougemont “Long-term suppression of tremor by chronic stimulation of the ventral intermediate thalamic nucleus”, The Lancet, Vol. 337, No. 8738, Feb. 16, 1991, pp. 403-406). In addition, stimulation of the subgenual cingulate cortex is being used experimentally for treating particularly severe forms of clinical depression that are resistant to treatment (H. S. Mayberg et al. “Deep brain stimulation for treatment-resistant depression”, Neuron, Vol. 45, pp. 651-660, Mar. 3, 2005). Stimulation is also being tested on the posterior hypothalamic nuclei for treating cluster headaches, on periaqueductal gray matter for attenuating pain, and on the ventromedial hypothalamus for treating certain kinds of obesity (A-L. Bernabid, B. Wallace, J. Mitofanis, C. Xia, B. Piallat, V. Fraix, A. Batir, P. Krack, P. Pollak, and F. Berger “Therapeutic electrical stimulation of the central nervous system”, Comptes Rendus Biologies, Vol. 328, No. 2, February 2005, pp. 177-186).
Under all circumstances, deep brain electrical stimulation involves inserting a flexible lead into the skull of the patient under guidance of a rigid stylet until the tip of said lead reaches the region of the brain that is to be stimulated. Close to its tip, the lead has electrodes or contacts (generally four in number), that are connected by a subcutaneous cable to a pulse generator, itself implanted under the skin of the patient, like a conventional cardiac pacemaker. After the introduction of the lead and its fixation to the patient's skull, the stylet which made the lead rigid and rectilinear, is extracted from the lead after serving for introducing the lead, and the lead can remain in place for a duration that may be as long as several years.
A more detailed description of the procedure of implanting a lead for deep brain electrical stimulation is given by the document “Medtronic—DBS™ Lead Kit For Deep Brain Stimulation 3387 3389—Implant manual”, from the supplier Medtronic Inc., which can be downloaded from the following Internet site: http://www.medtronic.com/physician/activa/downloadablefiles/197928_b—006.pdf.
By way of example, leads for deep brain electrical stimulation of conventional type are described in the above document from the supplier Medtronic, and also in U.S. Pat. No. 6,512,958.
The rectilinear shape of the lead is generally made necessary in order to ensure that insertion is as little traumatic as possible for the patient. Nevertheless, given the constraints that limit the insertion paths that can be selected for being followed by the lead without giving rise to unacceptable lesions in brain tissue, such a shape does not always make it easy to access regions that present therapeutic interest.
In addition, since a lead for deep brain electrical stimulation is implanted permanently for a duration of several years, there is a major risk of its tip becoming slightly displaced over time, thereby compromising its therapeutic effectiveness.
Document EP 1 062 973 describes a lead made of a flexible material and presenting a distal or head end of curved shape. The lead is inserted into the brain of a patient via a rigid cannula; while inside the cannula, said head end takes on the rectilinear shape of the cannula and returns to its curved configuration on coming out form the cannula at the end of the insertion operation. Such a lead is intended mainly for performing electrophysiological measurements prior implanting an electrical stimulation lead proper, with the curved shape of its head end enabling it to explore a plurality of brain zones without it being necessary to extract it and reinsert it several times over. However, it is less suitable for permanent implantation since it is difficult to extract the cannula alone from the patient's skull while leaving the lead in place. In addition, using an insertion cannula of outside diameter that is necessarily greater than that of the lead itself makes the implantation operation more traumatic.